Wind information can be reported by various sources (ATIS, METAR, TAF, spoken on the radio, etc). I was taught that officially some of these sources are relative to magnetic north and others to true north (although practically this isn't always followed). Which sources are supposed to use which reference system?

up vote 34 down vote accepted

The general rule is:

If you read it, it's true. If you hear it, it's magnetic.

All charts and textual sources (METAR, TAF, winds aloft, surface analysis charts, etc) use true north as the reference.

ATIS/AWOS/ASOS broadcasts, or any information a controller gives you over the radio, is magnetic.

Wind direction broadcast over FAA radios is in reference to magnetic north.

AIM Section 7-1-11 (page 7-1-26 in the 5/26/16 edition)

One exception to the "if you hear it" rule is that a FSS briefer will read you the winds referenced to true north, since they're just reading you the charts/textual information.

(This is at least true in the United States... other countries may vary in some instances)

  • 5
    It's the same in Europe. Wind information communicated by the tower when a landing/takeoff clearance is given is in degrees magnetic. METAR/TAFs are in degrees true. Not that it matters much because magnetic variation is almost zero in Western Europe – Philippe Leybaert Dec 18 '13 at 16:21
  • 3
    I believe one very important difference is TWEBs, or Transcribed Weather Broadcasts. Because they are an automated broadcast of a written report, they are reported in true headings, just like a written report, even though you technically hear it. – abelenky May 13 '16 at 20:07
  • Oops, I down-voted before I researched my answer completely. It appears we were both wrong. The AIM seems to indicate, as I expected, that the ASOS/AWOS data is the same as METAR, as I had expected. What I was wrong about them both being magnetic. – 2NinerRomeo Apr 4 '17 at 19:33
  • @2NinerRomeo you might want to research your answer... a little more completely ;) – Bret Copeland Apr 6 '17 at 3:53
  • Any idea what the reason is behind this? Seems like it would be easier/safer if everything was in one or the other. Thx! – Brian Armstrong May 14 '17 at 21:32

In the United States, the principal guide to weather products is AC 00-45G "Aviation Weather Services". As of this writing it appears Change 1 is current, published July 2010. Citations given here are to that revision. The Airman's information manual has information on AWOS and ASOS.

Here are some examples:

  • METAR: True 3.1.3.5
  • AWOS & ASOS: True AIM 7-1-11 Figure 7-1-5
  • TAF: True 7.2.2.5
  • WA (Winds Aloft): True 7.4.2
  • UA (Pilot Reports): Magnetic 3.2.1.9 (I'd suppose you get whatever the pilot reports.)
  • Surface Analysis Chart: True 5.1.2.3.5

When calling flight service, the briefer will have available the same information products that you can reach on aviationweather.gov. I expect they will know the direction reference for the information they are giving you.

  • When you say AWOS/ASOS to a pilot, they think of the automated radio report, where wind is given as magnetic, not true. I think you're getting mixed up because the ASOS equipment is also used to create METARs and other textual products, where wind is reported as true. Quote from nws.noaa.gov/asos/pdfs/aum-toc.pdf (section 3.2): "Wind direction is reported relative to true north in the METAR/SPECI message.... Wind direction is reported relative to magnetic north in the computer-generated voice messages" – Bret Copeland Apr 6 '17 at 3:37
  • Also, AC 00-45G is cancelled. AC 00-45H is current. – Bret Copeland Apr 6 '17 at 3:39
  • AIM Figure 7-1-5 is about decoding a textual METAR generated from ASOS/AWOS. It has nothing to do with the verbal radio broadcasts. See the note on page 7-1-26: "Note- Wind direction broadcast over FAA radios is in reference to magnetic north." (taken from the 5/26/16 AIM). – Bret Copeland Apr 6 '17 at 3:50

The way I remember it is that if it relates to an aerodrome its magnetic, because runway numbers are in magnetic and the pilot needs to know how the wind relates to the runway in use so the amount of headwind / crosswind can be assessed.

If it relates to en-route then its in true, because flight planning is done in true (with conversions to the correct magnetic heading to set on the compass at each step).

  • This is what I thought until I looked it up. I was gravely disappointed because this is how I thought it should be. – 2NinerRomeo Apr 5 '17 at 15:00

Another good general rule

Everything the weather man tells you it's true ... the rest is magnetic

  • This seems opposite to the previous answer. Are you talking about the TV weather man? That doesn't seem useful for aviators. – PJNoes Mar 16 '16 at 15:58
  • @PJNoes How did you get TV weather from this? It's an attempt to memorize that weather information is provided in True North, while operational (atis, tower) is Magnetic. "True" in this case meaning True North and not Accurate, Correct,etc.. hence the funny pun... well, attempt of, anyway... – Radu094 Mar 17 '16 at 12:37
  • I grew up when the only weather was from the weather man on TV. I got the funny pun, but the previous answer is easier for me to remember. No offence meant. – PJNoes Mar 17 '16 at 19:10
  • If the weatherman wrote it, its true. If he spoke it or tells it, its magnetic. Written is True. – abelenky May 13 '16 at 20:02
  • when is the weatherman speaking to you using magnetic north ? ... is what I'd like to understand .... – Radu094 Nov 11 '16 at 18:44

protected by SMS von der Tann May 13 '16 at 19:07

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.